See also topos theory.
The relationship XI (X is a subset of I) is a predicate. We can form a category of predicates on sets as follows:
predicates are pairs (I,X) where
X(i) implies an element iI is a free variable in I.
(I,X) -> (J,Y) where:
u:I-> J and X(i) implies: Y(u(i))
So, to define(I,X) -> (J,Y) , we need maps u:I -> J and X -> Y but X -> Y is implied by u.
'subsitution functor' u*
examples of substitution are: